1. Field of the Invention
The invention relates to piston rings for internal combustion engines and to methods for their manufacture.
The purpose of a piston ring of an internal combustion engine is to span the gap between the piston in which the ring is mounted and the associated cylinder or liner in order, during piston reciprocation, to prevent combustion gases passing between the piston and the cylinder and/or to control the thickness of the oil film on the associated cylinder or liner. In its simplest form, piston rings are of generally rectangular cross-section with the longer sides extending radially. This shape can be quickly and easily produced by a variety of well known methods.
2. Brief Review of the Prior Art
However, benefits have been shown to arise from the use of shaped piston rings. For example, it can be of benefit to provide one or two radially outwardly projecting rails on a piston ring, where the ring is controlling the thickness of the oil film on the associated cylinder or liner. Further, it has been found beneficial to provide a positive radially outward force acting on the ring in order to urge it towards the associated cylinder or liner and for this purpose annular springs have been provided acting on the radially inner surfaces of piston rings. For this purpose, it has been proposed to provide an annular recess extending around the piston ring in order to provide positive location of the spring on the piston ring.
These more complex shapes are more difficult to manufacture than the simple ring of rectangular cross-section. Many of the known techniques cannot be used. The most common method of producing such rings is by a moulding operation; for example a sintering and moulding operation, where the piston ring is of metal, or a hot pressing technique where the ring is of a thermosetting plastics material or an injection moulding process, where the ring is of a thermoplastic. However, moulding has the disadvantage that certain shapes can only be produced using complex equipment. For example, where re-entrant surfaces are required, it is necessary to use collapsible cores and this adds to the time, expense and cost of producing a piston ring. Such re-entrants may, for example, be found in the gaps between a pair of outwardly projecting rails or in a recess for a spring.